The present invention relates to an image edge correcting circuit and method for a digital image processor.
A general edge correcting circuit for an image in a television receiver is composed as shown in FIG. 1 and uses a correcting method to sharpen an edge of an image on a CRT which extracts outline components of an image luminance signal "Y" by passing a band pass filter or a high pass filter, mixes the extracted signal with a gain control signal in a multiplier (2), and then adds the image luminance signal "Y" to the mixed signal in an adder (3).
The process is hereinafter described with reference to the waveforms of FIG. 2.
When the image luminance signal "Y" is as in "2a" of FIG. 2, the extracted edge of the image luminance signal "Y" through the filter (1) is as in "2b" of FIG. 2. After the edge of the image luminance signal "Y" is extracted, the extracted edge components signal "2b" of the image luminance signal "Y" is mixed with the gain control signal by the multiplier (2), and then added to the image luminance signal "Y" at the adder (3) to output a final edge-corrected signal "2c". If the final edge-corrected signal as in "2c" is scanned on a CRT, a difference of the luminance signal between a dark portion and a bright portion is about "H2" which is more effective than "H1" the result of scanning the image luminance signal "Y" as is shown in "2a". Therefore, the general edge correcting circuit for an image which extracts an edge of an image helps viewers to enjoy a vivid image by increasing the contrast of the edge.
Also, by using a digital differential amplifier as in FIG. 3, the edge correcting circuit for an image stated above can become a digital edge correcting circuit. By sampling and quantizing the analog image luminance signal "Y", the analog image luminance signal Y' becomes a digital image luminance signal Y' as is shown in "4a". After the digital image luminance signal Y' of "4a" goes through a first delay circuit (102), the signal is delayed as in "4b", and the digital luminance signal "4b" passed through the first delay circuit (102) becomes as in "4c" after being outputted by a second delay circuit (103). A signal as in "4d" is obtained at a first subtractor "104" by subtracting the digital luminance signal "4a" from the first delayed digital luminance signal "4b", and a signal as in "4e" is obtained at a second subtractor (105) by subtracting the second delayed digital luminance signal "4c" from the first delayed digital luminance signal "4b". A signal "4f" is generated at a first adder (106) by adding the signal "4d" from the first subtractor (104) and the signal "4e" from the second subtractor (105). The signal "4f" from the first adder (106) is mixed with a gain control signal, by means of the multiplier (107), and then added to a signal from a delay circuit (101) at a second adder (108) to generate a edge-corrected digital luminance signal for an image "4g". The edge-corrected digital luminance signal for an image "4g" is converted to an analog signal as shown in the dotted line, by D/A a converter (not shown).
In the first edge-corrected circuit as in FIG. 1 or the second edge-corrected circuit as in FIG. 3, a delay-correction differs according to the constitution of a filter (1) or the first and second subtractors (104) and (105), which determined the frequency band for the edge-corrected circuit. Thus, the conventional method uses the image luminance signal for correcting a edge region. Hence, when the image is displayed on a CRT or a Display Unit, the conventional method sharpens a edge region by increasing a difference of level of a luminance signal at a boundary region, whether the circuit is used in an analog system or digital system.
However, when the stated conventional method is used to correct a edge of a bright boundary region where the signals represent white captions and capitals, an excessive current flows at the edge of a boundary region which results, in turn, in a serious drawback in that a blurring phenomenon results around the boundary region on the CRT display.